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June 27, 2012 -- Vitamin D deficiency has been linked to a host of illnesses and conditions, from heart disease and diabetes to certain types of cancer.

Now a small study of three women suggests that vitamin D deficiency and depression may travel together, and that filling up the "vitamin D tank" may help relieve some of the symptoms.

Whether low levels of vitamin D cause depression, worsen it, or are a symptom of the underlying depression is not fully understood. "There is no solid proof that vitamin D deficiency causes depression," says researcher Sonal Pathak, MD. She is an endocrinologist at Bay Health Endocrinology in Dover, Del. "Large studies are clearly needed."

Her findings were presented at The Endocrine Society's 94th Annual Meeting in Houston.

Pathak's study included three women aged 42 to 66 with depression, all of whom were taking antidepressants. The women were also being treated for type 2 diabetes or an underactive thyroid gland.

All three were deficient in vitamin D, with levels that ranged from 8.9 to 14.5 nanograms per milliliter of blood (ng/mL). Levels below 21 ng/mL are considered vitamin D deficient. Normal vitamin D levels are above 30 ng/mL, according to guidelines set by The Endocrine Society.

The women received vitamin D therapy for eight to 12 weeks to replenish their blood levels. After treatment, their levels increased to 32 to 38 ng/mL. The women also reported corresponding improvements in symptoms of depression following vitamin D therapy. One woman's depression score changed from one indicating severe depression to mild depression. Another woman's score improved to a level suggesting she had just minimal symptoms of depression.

Vitamin D is called the sunshine vitamin because our bodies produce it when exposed to sunlight. Vitamin D is also added to milk and other foods, and is available in small amounts in fatty fish like tuna, salmon, and mackerel; beef liver, cheese, and egg yolks. It can be hard to get as much as we need from our diets, which is why supplements are often needed.

The Institute of Medicine recently raised the recommended daily intake to 600 IU for people aged 1-70 and to 800 IU for adults older than 70. Other groups set the bar even higher.

The Link Between Vitamin D Levels and Depression

The relationship between depression and vitamin D is likely a two-way street, Pathuk says. "People who have depression are at high risk for vitamin D deficiency because they stay indoors, don't exercise too much, and are likely not eating a healthy diet."

There are also vitamin D receptors in areas of the brain that help regulate behavior and emotion, she says.

"It is not unusual for people with depression to be deficient in vitamin D and treating the deficiency may make a huge difference in how they feel," Pathuk says.

If you are being treated for depression, ask your doctor to test your vitamin D levels. "If you are deficient, get treated," she says.

"People often feel better when they take vitamin D," says Michael Holick, MD, PhD. He is the director of the Vitamin D, Skin, and Bone Research Lab at Boston University. "One of the effects that vitamin D has on the brain is to improve serotonin levels -- which is the same chemical that many antidepressants act on."

"This is an interesting study," says Erin LeBlanc, MD. She is an endocrinologist and researcher at the Kaiser Permanente Center for Health Research in Portland. "It does show that vitamin D and its effects on things besides bone should be studied more."

The next step would be a trial where some people with depression get vitamin D and others get an inactive placebo, she says.

These findings were presented at a medical conference. They should be considered preliminary as they have not yet undergone the "peer review" process, in which outside experts scrutinize the data prior to publication in a medical journal.